1. Field of the Invention
This invention relates to a mounting of a planetary gear assembly on a casing.
2. Description of the Prior Art
Planetary gear assemblies have a wide scope of utility as a decelerator or accelerator.
Planetary gear assemblies are able to be used in various manners. In most cases the outer inner-toothed gear is fixed on a casing. Namely, in these cases the outer inner-toothed gear is tightly fixed on the casing by screws parallel with the axis of the gear assembly.
Recently, planetary gear assemblies have increasingly been adapted as reduction devices e.g. for electric washers instead of belt-pully type reduction devices.
Japanese Patent Publication No. 60-27320 (published on June 28, 1985) disclosed a single-tub electric washer having a planetary gear assembly as the reduction device.
Because a planetary gear assembly can be used as an integrated unit, it enables one to simplify the manufacturing process of an electric washer.
In the prior art axially-directed screws tightly fix an outer inner-toothed gear on a casing. Because of the screw-fixation the outer inner-toothed gear is tightly coupled with the casing. Four screws or eight screws may be used in the fixation. However the screw-fixation takes much time. Namely, a worker must coincide the holes bored through the outer inner-toothed gear with the screw holes bored on the casing, insert screws into the coincided holes and revolve turn the screws with a screwdriver.
To simplify this process, more convenient fixation was devised. Several protrusions are formed in the axial direction on a casing, which correspond to the holes of the outer inner-toothed gear. By inserting the protrusions of the casing into the holes of the outer inner-toothed gear and pushing the outer inner-toothed gear by a ring-shaped pressing plate, the outer inner-toothed gear is tightly fixed on the casing.
There is no clearance between the holes of the outer inner-toothed gear and the protrusions of the casing. Thus the outer inner-toothed gear is firmly fixed without backlash. This fixation forbids relative movement between the casing and the outer inner-toothed gear.
However this fixation takes still much time, because a plurality of holes of the outer inner-toothed gear must be fitted on the corresponding protrusions.
In general, reduction gears generate noise from the engagement of gear teeth. Besides electric washers, the reduction gears employed in any machines generate noise more or less.
Of course another parts except the reduction gear also generate noise, because a machine consists of many movable parts.
Planetary gear assemblies have been well known for a long time. However they have hardly been used in practice as reduction devices because of the parctical defects. Because a planetary gear assembly has a high symmetry, its transmission efficiency ought to be high. But it is not always true.
Unlike other gear-train systems, the freedom of gear engagement is very low in a planetary gear assembly, because of the excess engagement points among gears. Small size errors of parts frequently generate the reaction forces which suppress the relative rotation of gears. This fact has been reducing the merit of planetary gear assembly.
Because this difficulty is able to be resolved by raising the precisions of gear-shaping, good planetary gear assemblies have been manufactured by assembling the gears finished carefully without size error.
Accordingly the conventional planetary gear assemblies must be highly expensive. This fact may suppress the pervation of planetary gear assemblies as reduction devices.
Thus it was thought that an outer inner-toothed gear should be tightly fixed on a casing with the least size error. Therefore the fixation by screws, protrusions and holes has been adopted for a long time for mounting a planetary gear assembly in a casing.
This Inventor has previously invented a novel planetary gear assembly which may invert the prevailing common sense (Japanese Patent Application No. 56-193113, Japanese Patent Laying Open No. 58-94656, laid open on June 4, 1983).
The novel planetary gear assembly has a plurality of planetary gears which consist of a ring-shaped gear and two side discs larger than the tooth-edge circle of the gear. The ring-shaped gear is sandwiched by two side discs.
In correspondence with the structure of the planetary gears, the outer inner-toothed gear consists of a gear part and two side inner-cylindrical parts larger than the tooth-root circle of the gear part.
Because the planetary side discs roll on the side inner-cylindrical parts of the outer inner-toothed gear, the radial displacements of the planetary gear are effectively suppressed, which ensures smooth rotations of gears.
The Inventor thinks that the planetary gear assembly is an unprecedented invention which apparently contradicts with the common sense of the field of technology.
The line velocity of gear is defined as a tangential speed on the pitch circle of gear.
A complex gear which has one or two side friction discs has been used from old to prevent an excess engagement of teeth.
The diameter of the side friction discs were equal to that of the pitch circles of the gears without exception. If the diameters of the side discs were different from the pitch circles, two line velocities of the side discs were different. To equalize the line velocities of the two discs, the diameter of the side discs must be equal to that of the pitch circles.
The Inventor has denied the common sense that the side discs must be same with the pitch circles in complex gears.
The novel planetary gear invented by this Inventor has side discs larger than the tooth-edge circle which is still larger than the pitch circle. The planetary gear assembly having the novel planetary gear can rotate despite the difference of the line velocities of the rolling discs. The reason why the gears can rotate will be explained now.
Of course the line velocities of the rolling side discs are different, but the difference is very little. This is one reason.
Little difference of line velocities can be cancelled by widening clearances between the discs and the gear parts, because the wide clearance allows slipping between the side discs and the gear parts. This is another reason.
This Inventor became aware of the two facts.
First the difference .DELTA.V of the line velocities of two side discs rolling with each other is given by EQU .DELTA.V =.delta..OMEGA. (1)
where .delta. is the deviation of the disc surface from the pitch circle and .OMEGA. is the angular velocity of the gear.
The difference .DELTA.V may be intuitively estimated as 2.delta..OMEGA., because both of the rolling discs and inner cylindrical parts deviate by .delta. from their pitch circles.
But this is wrong. A brief calculation shows us that the difference is only .delta..OMEGA..
Furthermore if wide clearances are formed between the outer surfaces of the discs and the inner-cylindrical parts, the planetary discs can easily slip against the inner cylindrical parts. The slipping cancels the difference of line velocities.
Besides, another wide clearances between the planetary disc and the planetary ring-shaped gear make the planetary discs slipping against the planetary gear.
The concept of wide clearances sharply contradicts the conventional common sense of planetary gear assembly.
By taking wide clearances between gears, discs and inner cylindrical parts, the demand for precise finishing is able to be alleviated. The planetary gear assembly can be constructed with cheap parts shaped with low precision. The low precision can be compensated by the wide clearances among the parts. Therefore the invention enables us to manufacture planetary gear assemblies at low cost.
By the invention the planetary gear assemblies begin pervading in various purposes. The invention is one of highly significant inventions in the history of planetary gear assemblies.
There are various modes of size errors in a planetary gear assembly. One error mode is the radial deviation of sun gear. Another error mode is the radial deviation of the carrier. These error modes are caused by the errors of radial displacements or angles of planetary shafts.
The degree to determine the precise positions of planetary shafts on a carrier is called "division precision". High division precision would ensure an excellent planetary gear assembly with low noise and high transmission efficiency.
However it is very difficult and expensive to heighten the division precision.
Furthermore an input shaft and an output shaft should be fitted to the sun gear and the carrier. The shafts are sustained by bearings on the casing. In many cases the center of the bearings deviates from the center of the casing.
Therefore there are various size errors of finishing parts, determining positions and assembling parts.
This Inventor thinks the conventional fixation mounting of an outer inner-toothed gear on a casing shall distrub the smooth rotation of gears, increases the tooth-edge interference and generate large noise, because size errors are unable to be cancelled in the case of fixation mounting.